1. Field of the Invention
This invention relates generally to the diagnostic testing of body fluids and, more particularly, to low-cost, disposable, ion-selective electrode assemblies for use in automated analysis systems.
2. Description of the Related Art
Electrode assemblies of this particular kind have special utility as part of infusion fluid delivery systems commonly used in hospital patient care. Such systems infuse nutrients, medications, and the like directly into the patient at a controlled rate and in precise quantities for maximum effectiveness. Infusion fluid delivery systems are connected to a patient at an intravenous (IV) port, in which a hollow needle/catheter combination is inserted into a blood vessel of the patient and thereafter an infusion fluid is introduced into the vessel at a controlled rate, typically using a peristaltic pump. Blood chemistry monitoring systems that are combined with infusion delivery systems of this kind use the IV port to periodically withdraw a blood sample, perform measurements of blood ion concentrations and the like, and then discard the blood or reinfuse it into the patient. The system then resumes delivery of the infusion fluid.
Such combined infusion fluid delivery and blood chemistry monitoring systems include an infusion line and catheter through which the infusion fluid is provided to the patient and blood samples are withdrawn. The infusion line incorporates an ion-monitoring electrode assembly having electrochemical sensors that are periodically exposed to the blood samples and thereby provide electrical signals to an analyzer for conversion into corresponding blood chemistry data. A control unit periodically halts delivery of the infusion fluid for a brief interval, during which time a blood sample is withdrawn from the patient into the infusion line and routed to the electrode assembly, which then generates the electrical signals. After the electrical signals have been received by the analyzer, the control unit disposes of the blood or reinfuses the blood sample into the patient, and the flow of infusion fluid is resumed.
The electrode assembly typically includes a reference electrode and a plurality of sensor electrodes that are each sensitive to a particular ion of interest. All of the electrodes are embedded in the base of the electrode assembly. Ion-sensitive electrodes generate electrical signals only in response to contact with the particular ion to which they are sensitive, and therefore provide selective measurement of the amount of that ion in the blood. Sensor electrodes can be provided to measure, for example, blood calcium, hydrogen ion, chloride, potassium, and sodium. In a differential measurement system, the reference electrode might be a pseudo-reference electrode (e.g., a chloride or sodium electrode) that is continuously exposed to a calibration or reference fluid. Alternatively, in a non-differential measurement system, the reference electrode can be exposed either to the reference fluid or to the blood while maintaining its fixed rest or steady-state potential.
In a differential measurement system, during the delivery of calibration fluid, the calibration fluid flows past both the reference electrode and the sensor electrodes, and the electrical potential between the reference electrode and each sensor electrode is measured. This provides a calibration measurement of the electrode assembly. During a subsequent blood chemistry measurement, a blood sample is drawn into the electrode assembly, where it comes into contact with the sensor electrodes, but not the reference electrode. The electrical potential between the reference electrode and each sensor electrode is measured again and compared with the earlier calibration measurement to provide an indication of the ion concentration in the blood of the particular ion to which the sensor electrode is sensitive. After measurement is completed, the blood sample is discarded or reinfused from the electrode assembly back into the patient, and delivery of infusion fluid is resumed.
The accuracy of measurement described above can be adversely affected by blood cells and other blood components (e.g., protein) that build up in the various spaces and crevices of the sensor assembly. That is, the accumulation of extraneous blood cells around the sensor electrodes can result in erroneous calibration and inaccurate measurement of blood chemistry. To avoid this problem, successive measurements generally cannot be taken unless the electrode assembly has been thoroughly purged of blood cells between measurements. Thus, a specified purge volume of infusion or calibration fluid must be passed through the electrode assembly after each measurement.
To reduce the blood fouling problem described above, it is known to provide a smooth flow path in the electrode assembly. Some assemblies, for example, provide flush-mounted electrodes, in which the electrodes are located in a sensor cavity and a water soluble reference material or gel is placed above the electrode and a polymer-based, selectively-permeable material is placed on top of the gel to form a smooth flow pathway for samples. The selectively-permeable material presents a smooth outer surface to the fluid flow, flush with the electrode housing, and it allows only selected ions to reach the reference gel and thereby produce a reading from the electrode.
Such flush-mounted electrode designs, however, can be difficult to manufacture. To provide accurate readings, a precise separation must be provided between aqueous-based internal reference gel and the polymer-based selectively permeable material, and the electrode must be kept free of any contact with the fluid being measured. For example, in the prior art electrode assembly illustrated in FIG. 1, a housing 110 having a cavity 112 with an electrode 114 is capped by a dry bead of reference gel material 116 and covered by a selectively-permeable membrane layer 118. The gel is a water-permeable material that can creep up the side walls 120 of the cavity. This is commonly referred to as wicking, and it can result in inaccurate measurement of blood chemistry. Accurate measurement of blood chemistry requires precise layer thickness of the membrane layer 118 and a good bond between that layer and the housing cavity 112. In the assembly of FIG. 1, wicking allows the reference gel material 116 to spread to areas of the cavity where the selectively-permeable layer 118 will be deposited. This weakens the bond between the housing and the selectively-permeable layer, and it can allow the layer to lift up and out of the cavity or allow the fluid being measured to travel along the cavity wall and reach the electrode, shorting the electrode out. Therefore, wicking also reduces the electrode assembly's shelf life.
Other factors besides the accumulation of blood cells around the electrode in the fluid path can also adversely affect the accuracy of the measurement. For example, the flow rate of the blood can influence the collection of blood cells or the reaction of the electrode during measurement. Instability in the rate of blood sample flow might influence the travel of ions to the electrode and induce inaccurate readings. In addition, patient comfort and ease of assembly replacement are important considerations in the selection of competing electrode assembly configurations. Conventional assemblies can be rather unwieldy, making them uncomfortable for patients and inconvenient for medical personnel in changing assemblies after use.
From the discussion above, it should be apparent that there is a need for an ion-monitoring electrode assembly useful, for example, in a combined infusion fluid delivery and blood chemistry measurement system that provides accurate, reliable measurements of blood chemistry, that avoids collection of blood cells in the assembly pathway, and that provides an increased shelf life and production yield, along with greater patient comfort and ease of use. The present invention satisfies this need.